digraph G {
  a[label="sort [4,2,6,1]", shape=rectangle];
  b[label="merge [2,4]", shape=rectangle, style=invis];
  c[label="merge [1,6]", shape=rectangle, style=invis];
  {rank=same; b,c}
  d[label="sort [4]", shape=rectangle, style=invis];
  e[label="sort [2]", shape=rectangle, style=invis];
  f[label="sort [6]", shape=rectangle, style=invis];
  g[label="sort [1]", shape=rectangle, style=invis];
  {rank=same; d,e,f,g};
  a -> b[style=invis];
  a -> c[style=invis];
  b -> d[style=invis];
  b -> e[style=invis];
  c -> f[style=invis];
  c -> g[style=invis];
}
